1. Field of the Invention
The present invention relates to an optical element having a birefringence functional layer, and in more detail, to an optical element having a birefringence functional layer formed by aligning and fixing a polymerizing liquid crystal monomer. Furthermore, the invention relates to a liquid crystal display device member and a liquid crystal display device which are provided with the optical element, a method of producing the optical element, and a method of evaluating the birefringence functional layer.
2. Description of the Related Art
A liquid crystal display device, having various advantages such as being easy to make thin and light, able to reduce the consumption power and difficult to cause flickers, is used in various fields such as televisions and medical equipments. The liquid crystal display device switches light by making use of the birefringence property of a driving liquid crystal material. Accordingly, the liquid crystal display device has the viewing angle dependency derived from the birefringence property of the driving liquid crystal material. In addition to the above, the liquid crystal display device has a problem in that, depending on an angle by which a viewer sees a liquid crystal display screen, light leakage or gradation reversal phenomenon is caused, that is, the viewing angle becomes narrower.
In order to overcome the problem, developed is a birefringence functional layer forming film having a retardation control function that controls exit light from a liquid crystal cell or incident light to a liquid crystal cell. The birefringence functional layer forming film is usually prepared by stretching a film such as polyacrylate, polycarbonate or triacetyl cellulose. The film is located outside of a liquid crystal cell formed by sandwiching a driving liquid crystal material with two substrates. At that time, the birefringence functional layer forming film is usually adhered to a substrate with an adhesive to use. However, since the refractive index of the adhesive coated on the substrate is different from the refractive index of a birefringence functional layer forming film, there is a problem in that the irregular reflection is caused at a display surface. Furthermore, there is a problem in that, when the birefringence functional layer forming film is adhered outside of the substrate, the exposed film absorbs moisture to expand. Still furthermore, since a birefringence functional layer forming film cannot be patterned in accordance with a pixel size of a display and the heat resistance is low, there is a problem in that the optical characteristics vary owing to the shrinkage with time.
In order to overcome the problems, there are recently proposed a method of forming a birefringence functional layer having a retardation control function from a liquid crystal material such as a polymerizable liquid crystal or a polymer liquid crystal, and a method in which the birefringence functional layers are arranged inside of two substrates that sandwich a driving liquid crystal material for use as a so-called in-cell type (for example, Japanese Patent Application National Publication No. 2004-524385 (JP-A-2004-524385)). When a birefringence functional layer is formed with the polymerizable liquid crystal material, it is necessary that after the polymerizable liquid crystal material is coated on a predetermined substrate, desired alignment is promoted and the alignment is fixed. JP-A-2004-524385 discloses an invention of an alignment layer having a retardation control function, which is formed by homeotropically aligning a polymerizable liquid crystal material. When a three-dimensional polymer network is formed in the alignment layer, as a method of increasing the crosslinking density of the polymer disclosed is a method where a non-mesogenic compound having at least two polymerizable functional groups is added to a polymerizable material.
A birefringence functional layer used as the in-cell type is excellent in that it can overcome problems possessed by the birefringence functional layer forming film of a type that is adhered outside of a substrate.
Usually, the birefringence functional layer, when formed in an optical element, excellently shows a desired alignment. However, when a different layer is further laminated on the optical element to form a liquid crystal device member, there is a problem in that the alignment of a polymerizable liquid crystal material that constitutes a birefringence functional layer is disturbed or the characteristics of the birefringence functional layer are disturbed. This results in another problem in that an optical element having such a birefringence functional layer cannot exhibit a high quality retardation control function.